In a DIAL system, the received back scattered signal is a function of: the transmitted laser pulse energy; the speed of light; the laser pulse width; the telescope area (field of view); the range (inverse square law); the offline beam and online beam overlap and the field of view (i.e. the geometric form factor); the spectral response of the receiver optics; the plume transmission; the total atmospheric transmission; and the ground cover type.
One use of a two-line DIAL system is to estimate the concentration path length (CPL) of a fluid related plume. Therefore, the online wavelength is desirably selected such that it is only absorbed by the target molecule of the fluid and nothing else in the optical path. The offline wavelength is desirably selected such that it is not absorbed by the target molecule. More desirably, the online and offline wavelengths are selected such that the ratio of the geometric form factor, the spectral response of the receiver optics, and the surface reflectivity corresponding to the online and offline wavelengths are approximately the same. As may be seen in Equation 1, when this condition is met, these parameters may cancel out, simplifying calculation of the CPL.
                              CPL          =                                                                      ln                  ⁡                                      (                                                                                            E                          ⁡                                                      (                                                                                          λ                                Off                                                            ,                              R                                                        )                                                                          ⁢                                                                              E                            1                                                    ⁡                                                      (                                                          λ                              On                                                        )                                                                          ⁢                                                  ξ                          ⁡                                                      (                                                          R                              On                                                        )                                                                          ⁢                                                  ξ                          ⁡                                                      (                                                          λ                              On                                                        )                                                                          ⁢                                                  ρ                          ⁡                                                      (                                                          λ                              On                                                        )                                                                                                                                                E                          ⁡                                                      (                                                                                          λ                                On                                                            ,                              R                                                        )                                                                          ⁢                                                                              E                            1                                                    ⁡                                                      (                                                          λ                              Off                                                        )                                                                          ⁢                                                  ξ                          ⁡                                                      (                                                          R                              Off                                                        )                                                                          ⁢                                                  ξ                          ⁡                                                      (                                                          λ                              Off                                                        )                                                                          ⁢                                                  ρ                          ⁡                                                      (                                                          λ                              Off                                                        )                                                                                                                )                                                  -                                                                  ⁢                                  2                  ⁢                                                            ∫                      0                      R                                        ⁢                                                                  (                                                                              k                            ⁡                                                          (                                                                                                λ                                  On                                                                ,                                r                                                            )                                                                                -                                                      k                            ⁡                                                          (                                                                                                λ                                  Off                                                                ,                                r                                                            )                                                                                                      )                                            ⁢                                                                                          ⁢                                              ⅆ                        r                                                                                                                        2                ⁢                                  (                                                            σ                      ⁡                                              (                                                  λ                          On                                                )                                                              -                                          σ                      ⁡                                              (                                                  λ                          Off                                                )                                                                              )                                                      -                          RC                              t                -                bag                                                    ,                            (                  Eq          .                                          ⁢          1                )            where λOn/Off is the online (or offline) peak wavelength, σ(λOn/Off) is the online (or offline) cross-section, E1(λOn/Off) is the online (or offline) transmitted laser pulse energy, R is the range/altitude/distance of the sensor to the target, E(λOn/Off,R) is the online (or offline) received laser pulse energy, ξ(ROn/Off) is the geometric form factor for the online (or offline) peak wavelength, ξ(λOn/Off) is the spectral response of the receiver optics for the online (or offline) peak wavelength, ρ(λOn/Off) is the background surface reflectance for the online (or offline) peak wavelength, k(λOn/Off,r) is the atmospheric attenuation coefficient for the online (or offline) peak wavelength, and Ct-bag is the target molecule concentration in the atmosphere.
In practice, however, when the geometric form factor, the spectral response of the receiver optics, and the surface reflectivity corresponding to the online and offline wavelengths are not approximately the same, then the values of these parameters for each online and offline measurement must be measured to accurately estimate the target plume CPL. The geometric form factor and spectral response of the receiver optics corresponding to the online and offline wavelengths may be measured and the DIAL system may be calibrated accordingly. Unfortunately, correcting for reflectivity variations due to ground surface cover type may be difficult in many situations. If these ground surface cover type reflectivity variations are not properly corrected, significant errors in CPL estimates of the target molecule may result, leading to false identification of plumes (or lack of plumes).
The present invention relates to an improved method of correcting for ground surface cover type reflectivity variations in DIAL measurements. Improved methods of the present invention may increase the probability of detection of plumes containing the target molecule. For example, these improved methods may be useful in identification of plumes generated by leaks in pipelines or storage tanks, plumes caused by spills and other contamination, and naturally occurring plumes such as gases emitted by volcanoes.